Shared Role for Differentially Methylated Domains of Imprinted Genes

Abstract

For most imprinted genes, a difference in expression between the maternal and paternal alleles is associated with a corresponding difference in DNA methylation that is localized to a differentially methylated domain (DMD). Removal of a gene's DMD leads to a loss of imprinting. These observations suggest that DMDs have a determinative role in genomic imprinting. To examine this possibility, we introduced sequences from the DMDs of the imprinted Igf2r, H19, and Snrpn genes into a nonimprinted derivative of the normally imprinted RSVIgmyc transgene, created by excising its own DMD. Hybrid transgenes with sequences from the Igf2r DMD2 were consistently imprinted, with the maternal allele being more methylated than the paternal allele. Only the repeated sequences within DMD2 were required for imprinting these transgenes. Hybrid transgenes containing H19 and Snrpn DMD sequences and ones containing sequences from the long terminal repeat of a murine intracisternal A particle retrotransposon were not imprinted. The Igf2r hybrid transgenes are comprised entirely of mouse genomic DNA and behave as endogenous imprinted genes in inbred wild-type and mutant mouse strains. These types of hybrid transgenes can be used to elucidate the functions of DMD sequences in genomic imprinting.At imprinted loci in mammalian species, the maternal and paternal alleles of a gene are distinguished from one another by different epigenetic modifications, genomic imprints, which are established during oogenesis and spermatogenesis. The transmission of these imprints is essential for normal embryonic development and leads to monoallelic gene expression in the embryo and adult (2, 11). The epigenetic modification that distinguishes the maternal and paternal alleles of most, if not all, imprinted genes is DNA cytosine methylation (10, 11, 23). The parental alleles have different levels of DNA methylation, usually concentrated in a single location within or surrounding the gene. A genomic region with this epigenetic feature is generally 1 to 5 kb in size and is called a differentially methylated domain (DMD). Within a DMD, one parental allele is highly methylated on the majority of CpG dinucleotides, and the opposite parental allele is unmethylated or methylated on a small percentage of CpG dinucleotides.The methylation patterns of several DMDs have been extensively studied, including DMD2 of the insulin-like growth factor type 2 receptor (Igf2r) gene, the DMD of the Snrpn gene, and the DMD of the H19 gene. DMD2 is found in the second intron of Igf2r, is approximately 3 kb in size, and contains 28 CpG dinucleotides (27). The DMD2 CpGs are methylated on the maternal allele and unmethylated on the paternal allele. The DMD of Snrpn includes promoter sequences, the entire first exon, and the first intron of the gene and is approximately 6 kb in size (6, 24). Like DMD2 of Igf2r, the DMD of Snrpn is highly methylated on the maternal allele and unmethylated on the paternal allele (6, 24). These differences in methylation are established during gametogenes...

Semen samples showing an increased rate of spermatozoa with imprinting errors have a negligible effect in the outcome of assisted reproduction techniques

“…Nevertheless, the biological significance that the presence of specific hypo-or hypermethylated CpGs has in the overall behavior of a region can be irrelevant if we consider that, as has been indicated in the literature, imprinted regions do not show homogenous values of methylation in the totality of CpGs, 28 an observation that was corroborated by the methylation values of our control population. Additionally, a higher incidence of syndromes caused by imprinting anomalies inherited from the paternal genome would be expected in offspring conceived by ART if the presence of few CpGs with abnormal methylation levels brought on alterations.…”

Identification of Ssm1b, a novel modifier of DNA methylation, and its expression during mouse embryogenesis

“…There are no obvious repeats in gpt as potential methylation targets (Reinhart et al, 2002). Thus, it is possible that Ssm1b recognizes a specific sequence within gpt that it binds to and initiates methylation, and endogenous targets might share some common features with gpt.…”

Elongation of theKcnq1ot1transcript is required for genomic imprinting of neighboring genes

“…However, as shown in Figure 8, the loss of these 5Ј repeated sequences does not affect the imprinting of any of the genes in the cluster. Thus, although repeated sequences seem to be a common feature of imprinting gene domains, in only two instances have they been shown to play a mechanistic role (Reinhart et al 2002;Yoon et al 2002).…”